1. Field of the Invention:
This invention relates to a refrigerating system for generating low temperatures and comprising two systems of refrigerators, one for a high-temperature region and one for a low-temperature region.
More particularly, the invention relates to a refrigerating system for absorbing heat energy at low temperatures less than the critical temperature of a working medium, and comprises, in combination, a single low-temperature region refrigerator constituted by a compression space, a cooler, a regenerator and an expansion space, or a plurality of low-temperature region refrigerators each constituted by a compression space, a cooler, a regenerator, a heat exchanger and an expansion space (e.g. a Sterling cycle refrigerator, Vuilleumier cycle refrigerator, Gifford cycle refrigerator, Gifford-McMahon cycle refrigerator, etc.), and a high-temperature region refrigerator (e.g. a Stirling cycle refrigerator, Gifford-McMahon cycle refrigerator, Solvey cycle refrigerator, Vuilleumier cycle refrigerator, Claude cycle refrigerator, etc.). The invention is utilized in, for example, a cooling system mounted on a cryostat accommodating a liquid helium-cooled superconductor magnet for reliquifying helium vapor resulting from vaporization of the liquid helium due to heat penetrating the cryostat, whereby the amount of liquid helium within the cryostat is held constant at all times.
2. Description of the Prior Art:
A refrigerating system of the type to which the present invention appertains is disclosed in the specification of U.S. Pat. No. 4,335,579 and the specification of Japanese Patent Publication No. 51-13900.
The former is illustrated in FIG. 6 and is equipped with a high-temperature region refrigerator having a crankshaft 202 rotated by a power source 201, a piston 203 slidably reciprocated by the crankshaft 202, expansion spaces 204, 205, and low-temperature portions 206, 207, and with a low-temperature region refrigerator having a crankshaft 209 rotated by a power source 208, pistons 210, 211 slidably reciprocated by the crankshaft 209, a compression space 212, an expansion space 213, a heat radiating portion 214, and a regenerator 215. The heat radiating portion 214 is thermally coupled with the low-temperature portion 207, which absorbs the compression heat of the working medium generated at the compression space 212 of the low-temperature refrigerator. The low-temperature region refrigerator has a compression cylinder 216 and an expansion cylinder 217 that are thermally coupled via the low-temperature portion 206 and a precooling plate 218 to reduce heat flowing into the compression space 212 and expansion space 213 from portions at ordinary temperature.
The invention disclosed in the specification of Japanese Patent Publication No. 51-13900 relates to a method of absorbing heat energy at low temperature, as indicated by the T-S diagram (taking helium as an example) illustrated in FIG. 2. Specifically, in a refrigerator comprising a compression space, a heat radiating portion, a heat exchange portion (e.g. a regenerator or heat exchanger) and an expansion space, the disclosed method is directed to maintaining the pressure of the working medium above a pressure approximately equal to the critical pressure (Pc in FIG. 2)) at all times, and bringing the temperature of the expansion portion below the critical temperature (Tc in FIG. 2)) of the working medium. An absorbed amount of heat Q.sub.E generated by working medium expansion work in the expansion space, and an amount of mechanical work W applied to the working medium from the outside for the purpose of obtaining this heat absorption, are expressed as areas bounded by a.sub.2 a'.sub.2 a'.sub.3 a.sub.3 and a.sub.1 a.sub.2 a.sub.3 a.sub.4, respectively, as shown in FIG. 2.
The prior art described above involves a number of disadvantages and difficulties which will now be set forth.
In the refrigerating system of FIG. 6, the compression cylinder 216 and expansion cylinder 217 are cooled by the low-temperature portion 206 of the high-temperature region refrigerator via the heat radiating plate 218. Nevertheless, when the temperature of the compression space 212 is at the 10K level and the temperature of the expansion space 213 is at the 4K level, by way of example, the temperature of the low-temperature portion 206 is at the 20K level and the compression space 212 and expansion space 213 are penetrated by several Watts and 0.5 Watt of heat, respectively, as typical values. Consequently, the overall efficiency of the apparatus is poor, it is necessary to apply a large input to the entirety of the low-temperature apparatus in order to obtain the same refrigeration output at the expansion space 213 of the low-temperature region refrigerator, and the apparatus is of great size and weight. The reason is that since the compression space 212 and expansion space 213 are defined in the cylinders 216, 217 by the slidably disposed pistons 211, 210, respectively, the working medium invades a gap, which is formed between the piston and respective cylinder to permit the piston to slide, so that the heat carried by the working medium from the ordinary temperature portions and the heat transmitted through the piston cannot be reduced sufficiently.
The drawback with the method described in the specification of Japanese Patent Publication No. 51-13900 is that heat energy cannot be absorbed efficiently at a low temperature which is less than the critical temperature of the working medium.
The cause of this difficulty is failure to achieve a change in the state of the working medium. Specifically, in the refrigerator comprising the compression space, heat radiating portion, heat exchange portion (e.g. regenerator or heat exchanger) and expansion space, the pressure of the working medium is maintained above a pressure approximately equal to the critical pressure at all times and, consequently, when the working medium is expanded and made to absorb heat at a temperature below the critical temperature, no change of state occurs in the working medium. The amount of work W externally applied is distorted very minutely in the low-temperature region below the vicinity of the critical temperature near the critical pressure in the T-S diagram. As a result, there is a reduction in the amount of heat Q.sub.E absorbed.
It may thus be understood that COP (achieved efficiency) (=Q.sub.E /W), which represents the efficiency at which heat is absorbed, undergoes a significant reduction. Let us consider an example. For a working medium which is helium gas, a maximum pressure of 3 atm, a pressure ratio of 3, a compression portion temperature of 10K and an expansion portion temperature of 4.2K, COP will be only about 12%.